Confused about @foreign_key_type, can't find docs?

Hello, I have seen quite a few mentions of @foreign_key_type combined with e.g. a string primary key, but I can’t really understand its working and can’t find any docs about it?
As far as I can tell it applies wholesale to all foreign keys in the schema where the directive is defined, rather than where said schema itself is used as a foreign relation. I hope I made myself clear (see below), but if is the case, this makes no sense to me. What did I miss?

Example:

defmodule Book do
  ....
  @primary_key {:id, :string, autogenerate: false}
  @foreign_key_type :string

  schema "books" do
    belongs_to :author, Author # now :author_id is expected to be string?
  end
end

defmodule Page do
  ....
  schema "pages" do
    belongs_to :book, Book # I would expect :book_id to be of type string now
  end
end
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You would also need to set the @foreign_key_type (as well as @primary_key_type) in your Page module since you are not using the default key type of :id

https://hexdocs.pm/ecto/Ecto.Schema.html

Thanks, but if you read you right this implies the entire database uses the same type of primary key, which is not the case for me. The example is fictitious, but imagine the Book schema has an ISBN string as pk, while Page uses normal integers.

I must conclude the @foreign_key_type directive is only useful when the entire database uses a non-standard pk type.

Yes, exactly. It is better to pass the foreign_key_type explicitly for each association otherwise.

2 Likes

Great, thanks for clearing that up. BTW, the reason I complained about the docs is when I type foreign_key in the hexdocs search box it never shows the page this directive is described on? I think it would be more useful if it were a proper full text search.

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Yes, there is someone working on it but it is a non trivial problem. :slight_smile:

1 Like